Blood pressure

Cardiovascular Laboratory Blood pressure can be measured by several techniques.  The direct method (refer to the diagram below) involves directly inserting a tube or catheter into a blood vessel.   The catheter is connected to a blood pressure transducer, which generates an electrical signal.

In this experiment, we measure the arterial blood pressure using two different methods. Both of these methods are indirect, in that they do not involve inserting a catheter directly into the artery and connecting that catheter to a blood pressure transducer. The first method uses the sense of touch: it is thus called the palpatory method. The second method uses the sense of hearing: it is thus called the auscultatory method

The figure to the left shows a typical tracing of the blood pressure recorded from an artery using the direct method.  The maximum pressure is called the systolic pressure; the minimum pressure is called the diastolic pressure.   The pulse pressure is the difference between the systolic and diastolic pressures.  The mean pressure is given approximately by the sum of the diastolic pressure and one third of the pulse pressure

A sphygmomanometer, an instrument that measures pressure, is needed in both methods.   Each sphygmomanometer consists of a cuff (containing a "bladder") which is connected by lengths of tubing to an inflating bulb with a needle valve and to an aneroid pressure gauge. An appropriately sized cuff covers 2/3 of the biceps; the bladder is long enough to encircle >80% of the upper arm, and bladder width equals at least 40% of the upper arm's circumference.

Correct positioning of the cuff The centre of the sphygmomanometer bladder should be placed over the brachial artery. Many cuffs have some sort of marking scheme so that placement over the brachial artery - under, or just medial to, the biceps tendon - is facilitated.  The lower border of the cuff should be ~2cm proximal to the antecubital fossa and the cuff should be firmly wrapped around the arm.

Blood Pressure> Palpatory Method The relaxed subject sits on a chair.  The cuff of the sphygmomanometer is wrapped firmly around the right arm above the elbow. The lower arm should be resting on a table-top or bench.  The radial pulse (the pulse at the radial artery in the wrist) is palpated with the fingers of the left hand.  The number of beats in 30 seconds is counted, and the heart rate in beats per minute is recorded.

The valve on the inflating bulb of the sphygmomanometer is turned fully clockwise so that it is closed.  The cuff is inflated slowly (10 mm Hg/sec) by pumping the inflating bulb until the radial pulse is no longer felt.  The cuff is inflated further until the pressure is about 30 mm Hg higher. 

The valve on the inflating bulb is opened slightly by turning it in the counterclockwise direction, allowing the pressure to drop slowly by about 5 mm Hg/sec.  At some point, one will be able to feel the radial pulse once again.  The pressure indicated on the gauge when the pulse reappears is noted.  This is the systolic pressure.  Now the pressure in the cuff is quickly released, so as not to cause undue discomfort to the subject. The determination of blood pressure is repeated two more times for the right arm, and then repeated again using the left arm instead of the right arm

Explanatory Notes   The cuff pressure's constriction of the artery is opposed by the blood pressure.  Therefore, in order to completely constrict the artery, the cuff pressure must be greater than the highest blood pressure, the systolic pressure.

The cuff pressure is gradually decreased The cuff pressure is gradually decreased. At the point where the cuff pressure drops below the systolic pressure of the blood, blood can pass through the constricted opening of the artery and the pulse will be felt.  In other words, the systolic pressure, the highest blood pressure, will be the point at which the cuff pressure is first overcome.

The pulse will continue to be felt as the pressure in the cuff falls from systolic all the way down to zero.

Note that the although the radial pulse is palpated, the systolic blood pressure actually recorded is that in the brachial artery, where the actual cuff constriction occurs.

Auscultatory Method The relaxed subject sits on a chair with the lower arm supported as before.  The blood pressure cuff is placed on the subject's right arm, allowing 1 inch between the bottom of the cuff and the crease of the elbow.  The brachial pulse is palpated just above the angle of the elbow (the "antecubital fossa"). 

One group member puts on a stethoscope, with the earpieces on the headpiece angled forward. The recording end of the stethoscope is twisted, so that the diaphragm and not the bell is activated.  This can be tested by tapping lightly on the diaphragm. The diaphragm is placed over the brachial artery in the space between the bottom of the cuff and the crease of the elbow. At this point no sounds should be heard. 

The cuff pressure is inflated quickly to a pressure about 30 mm Hg higher than the systolic pressure determined by the method of palpation.  Then the air is let out of the cuff at a rate such that cuff pressure falls at a rate of about 5 mm Hg/sec.  At some point the person listening with the stethoscope will begin to hear sounds with each heartbeat.  This point marks the systolic pressure.  The sounds are called Korotkoff sounds

  As the pressure is lowered further, the character of the Korotkoff sounds should change.  At some point, the sounds will disappear.  The pressure reading at this point gives the diastolic pressure. The subject should now lie on his or her back for five minutes.  The systolic pressure and diastolic pressure are recorded.   Then the subject stands up, and the pressures are immediately recorded once more.

Explanatory Notes The laminar flow that normally occurs in arteries produces little vibration of the arterial wall and therefore no sounds.  However, when an artery is partially constricted, blood flow becomes turbulent, causing the artery to vibrate and produce sounds

When measuring blood pressure using the auscultation method, turbulent blood flow will occur when the cuff pressure is greater than the diastolic pressure and less than the systolic pressure.  The   "tapping" sounds associated with the turbulent flow are known as Korotkoff sounds. Remember that these sounds are not to be confused with the heart sounds produced by the opening and closing of the heart valves.

Summary of the auscultatory method: Initially the cuff is inflated to a level higher than the systolic pressure.  Thus the artery is completely compressed, there is no blood flow, and no sounds are heard.  The cuff pressure is slowly decreased.  At the point where the systolic pressure exceeds the cuff pressure, the Korotkoff sounds are first heard and blood passes in turbulent flow through the partially constricted artery. Korotkoff sounds will continue to be heard as the cuff pressure is further lowered.  However,  when the cuff pressure reaches diastolic pressure, the sounds disappear.  Now at all points in time during the cardiac cycle, the blood pressure is greater than the cuff pressure, and the artery remains open.

Errors in blood pressure readings: The cuff is not of the proper size: if the cuff is too small the blood pressure readings may be artefactually high. If the cuff is too big, the readings may be artefactually low. The cuff is positioned too loosely: the blood pressure may be artefactually high. The centre of the cuff bladder is not positioned over the brachial artery. The cuff is inflated slowly: a slow inflation causes venous congestion, which in turn causes the Korotkoff sounds to be faint; this results in false readings with the systolic value being too low and the diastolic reading too high. If the cuff is re-inflated immediately after an initial reading (trying to re-check the reading): a rapid re-inflation could cause venous distension, the Korotkoff sounds become more muffled. The initial Korotkoff sound may be missed so the systolic reading would be falsely low, and the diastolic reading would be falsely high because the last Korotkoff sounds could not be heard.